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Thursday, 17th April 2014

Using 9-day weather forecast smartly

The Observatory rolled out its 9-day weather forecast service on 1 April this year. The period of forecast was extended by two days from the previous 7-day forecast. This enables the public to appreciate weather changes earlier so as to plan their activities ahead and get well prepared. You may notice that the webpage was given a facelift. It displays the forecast of temperature and humidity on colourful charts and incorporates related climatological information. Besides freshening up the look, these features aid the appreciation of weather changes and comparison with past climate.

You may find some red and blue shadings on the temperature and humidity charts. Can you guess what they are? They represent climatological information in the past. For example, the red shading on the chart represents the middle 50% range of the daily maximum temperature or daily maximum humidity within 30 years from 1981 to 2010 arranged in ascending order. Let us take 17 April as an example. We choose a five-day period from two days before to two days after 17 April, i.e. from 15 April to 19 April. We then select the same dates over the 30-year period, giving a total of 150 days. The maximum temperature or maximum relative humidity of these days were then sorted in ascending order and divided into 100 equal parts statistically. The value corresponding to the part ranked 25 is called 25th percentile, while that corresponding to the part ranked 75 is called 75th percentile. The red shading represents the range between these two percentiles, which accounts for 50% of the whole set of data. Therefore, if the forecast value falls inside the red shading, it means that the value is within the middle 50% range of the climatological data. If the forecast value is above the red shading, it is higher than the middle 50% range of the climatological data. Similarly, the forecast value is lower than the range if it is below the red shading.

By the same token, the blue shading on the charts represents the middle 50% range of climatological data of daily minimum temperature or daily minimum relative humidity.

Figure 1

Charts showing temperature forecast, humidity forecast and climatological information.

The accuracy of forecast generally decreases with forecast period. The formulation of weather forecast for the eighth and ninth days is mainly based on the output from numerical weather prediction models. The accuracy also varies for different weather systems in different seasons. Generally speaking, weather in spring and summer such as fog, tropical cyclone, thunderstorm and heavy rain is more changeable, resulting in higher uncertainty of forecast. Objective verification of various weather elements shows that the average accuracy of the weather forecast for the eighth and ninth days reaches about 80 per cent, which is comparable to that of the sixth and seventh-day forecasts newly launched back in 2004.

Figure 2

Accuracy of forecasts for the 8th and 9th days on par with that of the 6th and 7th-day
forecasts newly launched back in 2004.

When you check the 9-day weather forecast next time, other than the forecast values of temperature and humidity, please also take a look at their day-to-day variation and trend, and compare them with the climatological data. When the day of concern comes close, remember to check the latest weather forecast by the Observatory!

F.Y. Lee


Wednesday, 26th March 2014

Clear Air Turbulence - CAT Encountering in the Air

Aircrafts in the air aloft may sometimes experience marked bumps and jolts and people would normally say "encountering an airstream". In meteorology, we call it "turbulence". When encountering turbulence, pilot would promptly switch on the seat-belt sign and passengers should follow the instruction to fasten the seat belt to protect their own safety. Under severe turbulence situation, an aircraft may experience intense vibration. It may also suffer from a momentary loss of control. Hence, pilots will normally avoid flying into the turbulence regions.

Turbulence is usually caused by convective weather, especially in the vicinity of thunderstorms. As aircrafts are generally equipped with weather radar which is capable of detecting clouds and weather associated with convection, pilots normally have sufficient time to diverge the aircraft to avoid impact. Even in case that there is no way to perform avoidance actions, pilots could still be able to alert in time the passengers to fasten the seat belt for safety.

Figure 1

Figure 1      Turbulence encountered by an aircraft in clear air area is named Clear Air Turbulence.

Turbulence, however, may also happen under clear sky. In the aviation community, this kind of turbulence is called "Clear Air Turbulence", or simply "CAT". As the literal meaning tells, CAT happens in regions of fine weather. As there is no rain water, CAT is not detectable by the radar weather onboard. If an aircraft so unfortunately has flown into a CAT region, passengers and crew members might get injured in sudden bumps and jolts. Around noon on 18 February this year, an inbound flight from San Francisco encountered severe turbulence when flying over Russia. There were reports that some passengers were lifted from the seats and bumped onto the overhead lockers or hit by falling luggage, causing 12 injuries. Fortunately, the aircraft was not damaged and landed safely at the Hong Kong International Airport (HKIA) at dusk.

Figure 2

Figure 2      Using computer simulation, we can forecast the location and development of jet stream in the
upper air (red colour curves). This figure indicates a jet stream occurred over Russia on 18 Feb 2014.
An aircraft experienced severe turbulence when flying (black colour straight line) near the jet stream.
(Source: World Area Forecast Centre - London).

Typical CAT often happens high in the sky at an altitude of 20,000 feet (i.e. 6 kilometres) or above. It is usually accompanied with high wind (known as "jetstream"), or regions of abrupt wind speed or direction changes (such as interface of cold and warm air masses, regions with airstream accelerating/decelerating or turning). CAT can also be caused by terrain, particularly waves triggered by airflow passing over high mountains. Taking the Hong Kong Flight Information Region (HKFIR) as an example, the frequency of aircraft encountering CAT was around 15 days per year on average, among which one day would be severe CAT. CAT mostly occurs during winter months from December to February. Turbulence associated with CAT constitutes as large as 10% of the total number of turbulence reports. Due to its hazardous characteristics, we should never overlook CAT. But, how can the threat of CAT be minimized?

In order to assess the chance of CAT and the location of its occurrence within HKFIR so as to issue timely alert to the pilot for making avoidance, the Observatory's aviation forecasters make reference to the observational data collected by weather balloons, meteorological satellites and the "CAT index" generated by computer-based numerical weather prediction models, taking also into consideration of the pilot reports. The forecasters would also outline regions with possible turbulence on significant weather charts for pilots' attention. Before take-off, all pilots should have received a flight document containing, inter alia, detailed weather forecasts pertinent to the flight route. As such, pilots would be on the alert of the regions with possible CAT along the route and take special precaution when they fly through those regions.

To monitor CAT in the vicinity of the airport at Chek Lap Kok, aviation forecasters rely mostly on LIDAR (Light Detection And Ranging). Simply speaking, LIDAR utilizes infrared laser beam to directly probe the wind distribution in clear air to detect any CAT occurrence. The LIDAR is indispensable to the windshear and turbulence alerting services at the HKIA. Since LIDAR is so effective in detecting CAT, if on one day in the future all aircrafts can be equipped with LIDAR onboard, CAT can be detected inflight. Thus, pilots can respond swiftly and alert following aircrafts so that the injuries due to CAT could be significantly reduced. As of the present reality, one should better "fasten the seat belt" for safety whenever onboard!

A Video on "CAT" (in Cantonese only) can be found in the Observatory's "Cool Met Stuff" on YouTube.

Statistics on the occurrence and background information about "CAT" in the HKFIR are available on the Aviation Weather Services webpage of the Observatory.

P.W. Li and P. Cheung


Wednesday, 29th January 2014

Warm Arctic, Cold Continents

Many parts of the world experienced extreme weather as the planet Earth continued its seasonal march into the new year of 2014 [1]. Winter in the Northern Hemisphere was a wildly fluctuating one (Figure 1). Blasted by frigid air from the polar region, temperatures in many states in the central, eastern and southern parts of the United States were well below freezing. Yet Alaska and the Arctic were abnormally warm. Much of Europe was also exceptionally mild in early January, with France reporting daily maximum temperatures five to nine degrees above the normal values of January. Summer in the Southern Hemisphere, on the other hand, was sizzling hot. The central and eastern interior of Australia was under the grip of a heat wave in early January, and this after Australia reporting a record hot year in 2013.

Figure 1

Figure 1      Air temperature (near ground) anomalies for 5-7 January 2014 over the Northern Hemisphere
(the Arctic region at centre) relative to the 1981-2010 average; blue is colder and red is warmer
(source: US National Oceanic and Atmospheric Administration).

It is recognized that climate change can lead to anomalous variations in weather patterns and the likelihood of more extreme weather. Some studies suggest that a warming climate can actually expose the mid-latitude region of the Northern Hemisphere to more incursion of cold air from the Arctic. While this may sound self-contradictory, there is actually a good physical reason behind.

Warming causes the melting of Arctic sea ice and snow cover on land areas, altering the reflectivity of the Earth. Sea ice and snow have much higher reflectivity than ocean and land, and hence are very effective in reflecting sunlight back to space. However, the declining sea ice and snow cover will expose more ocean and land surfaces, increasing the Earth's ability to absorb solar energy. The additional heat will warm the ocean and land, prompting further melting of sea ice and snow cover in a vicious cycle. As a result, the Arctic warms faster than elsewhere in the Northern Hemisphere. The temperature contrast between the polar region and the tropics is reduced, causing the westerly airstream aloft in the Northern Hemisphere to slow down and become more susceptible to wavy meanders. On the west flank of the wave, cold air from the polar region is drawn southwards by the northerly airstream; on the east flank of the wave, warm air from the tropics under a southerly airstream pushes towards regions further north (Figure 2).

Figure 2

Figure 2      Winds aloft (about 5 km above ground, unit of wind speed in m/s and indicated by shading) in the Northern Hemisphere
on 5-7 January 2014 (source: US National Oceanic and Atmospheric Administration).

The slow-down of the westerly airstream aloft also favours the establishment of atmospheric blocking in the form of slow-moving anticyclones. The evolution of weather systems then becomes locked into a certain pattern in time and space. Depending on where you are with respect to the blocking set-up, some locations may experience prolonged fine weather, while others may be trapped under a storm corridor in a spell of rainy and gloomy conditions. If blocking occurs in winter, unusually cold or mild weather may persist for days or weeks over the affected regions. One example in recent years that comes to mind is the cold spell experienced by most part of China in early 2008, with cold air from Siberia taking advantage of the time afforded by a blocking situation to spread incessantly southwards all the way to Hong Kong.

Of course, for one single extreme weather event, we can never be sure that it is attributable to climate change. Only a collection of such events in some kind of trends over a period of time can tell the full story. As such, there is always a temptation to just maintain a watching brief, in the hope that the climatologists may turn out to be wrong or, failing that, in the misguided belief that the human race always has the capacity to adapt to climate changes, especially if the full impact we are talking about is, may be, a hundred years or so down the line. But what if the fury of climate change can also be manifested in the form of more frequent extreme weather? We certainly do not want to react too late if the next extreme weather event to hit us is just around the corner!

S M Lee, H W Tong


[1] Extreme weather in parts of the world


Last revision date: <17 Apr 2014>